An NOx occluding and reducing catalyst has been known which occludes nitrogen oxides (NOx) in the exhaust gas by absorption or adsorption when the air-fuel ratio of the exhaust gas flowing in is lean, and reduces and purifies the NOx which it has occluded with a reducing component such as CO or HC component in the exhaust gas when the air-fuel ratio of the exhaust gas has turned into the stoichiometric air-fuel ratio or a rich air-fuel ratio. When the NOx occluding and reducing catalyst is used for purifying the exhaust gas, it becomes necessary to conduct the operation at a rich air-fuel ratio by changing the operating air-fuel ratio over to a rich air-fuel ratio for a short period of time every time when the amount of NOx occluded in the NOx occluding and reducing catalyst has increased during the operation at a lean air-fuel ratio, to thereby execute a rich spike operation for reducing and purifying NOx occluded by the catalyst by supplying the exhaust gas of a rich air-fuel ratio to the NOx occluding and reducing catalyst.
However, the rich spike operation is accompanied by the operation for changing the operating air-fuel ratio from a lean air-fuel ratio over to a rich air-fuel ratio by increasing the amount of supplying the fuel into the engine for a short period of time. Therefore, if the rich spike operation is executed for unnecessarily long periods of time or too frequently, the fuel is consumed in increased amounts by the engine and this causes a problem in that excess of HC and CO components that were not used for reducing and purifying NOx are released into the air.
Therefore, it becomes necessary to efficiently execute the rich spike operation for the NOx occluding and reducing catalyst. A device for purifying exhaust gases taught in, for example, JP-A-2003-56379 utilizes the output of an air-fuel ratio sensor provided in an exhaust gas passage downstream of the NOx occluding and reducing catalyst in order to efficiently execute the rich spike operation for the NOx occluding and reducing catalyst.
That is, according to the device disclosed in JP-A-2003-56379, the air-fuel ratio sensor is provided in the exhaust gas passage downstream of the NOx occluding and reducing catalyst to detect the air-fuel ratio of the exhaust gas, and the rich spike operation is terminated when the output of the air-fuel ratio sensor has changed from a value corresponding to a lean air-fuel ratio near the stoichiometric air-fuel ratio over to a value corresponding to a rich air-fuel ratio during the rich spike operation.
When the exhaust gas of a rich air-fuel ratio is supplied to the NOx occluding and reducing catalyst during the rich spike operation, NOx that is occluded reacts with HC and CO components in the exhaust gas on the NOx occluding and reducing catalyst, whereby HC and CO components rob NOx of oxygen to reduce NOx. Therefore, while the reaction for reducing NOx is taking place on the NOx occluding and reducing catalyst, HC and CO components in the exhaust gas are oxidized on the NOx occluding and reducing catalyst, and the air-fuel ratio of the exhaust gas from the NOx occluding and reducing catalyst acquires a stoichiometric air-fuel ratio (actually, an air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio).
On the other hand, when NOx occluded in the NOx occluding and reducing catalyst is all reduced, HC and CO components in the exhaust gas are no longer oxidized on the NOx occluding and reducing catalyst, and the air-fuel ratio of the exhaust gas from the NOx occluding and reducing catalyst quickly changes over to the side of the rich air-fuel ratio.
According to the device taught in JP-A-2003-56379, the rich spike operation is terminated when the output of the air-fuel ratio sensor on the downstream side becomes a predetermined judging value during the rich spike operation, and the judging value is varied in accordance with the operating conditions of the engine to optimize the moment for terminating the rich spike operation in accordance with the operating conditions of the engine.
In recent years, there has been used an engine equipped with a supercharger (so-called supercharged lean burn engine) which effects the supercharging even in the operation at a lean air-fuel ratio. In the supercharged lean burn engine, the supercharging is effected during the operation at a lean air-fuel ratio and the cylinders are filled with much air. Upon conducting the supercharged lean burn operation, therefore, it is allowed to maintain a lean air-fuel ratio while increasing the engine output by supplying fuel in an increased amount to the engine, making it possible to expand the region of lean air-fuel ratio up to a high-load region where the operation could not be conducted at a lean air-fuel ratio with conventional naturally-aspirated engines.
In the case of the supercharged lean burn engine, however, the amounts of air filled in the cylinders due to supercharging become large causing a problem at the time of executing the rich spike operation for the NOx occluding and reducing catalyst.
In the supercharged lean burn engine, for example, the amounts of air filled in the cylinders are larger than those of the naturally aspirated engines and, therefore, the amounts of exhaust gas increase. Therefore, the SV value (space velocity value) through the catalyst increases correspondingly, and some of the HC and CO components in the exhaust gas flow out to the downstream of the catalyst in increased amounts without reacting with the NOx on the catalyst; i.e., the so-called “blow-by” takes place.
When the “blow-by” takes place as described above, the NOx occluding and reducing catalyst becomes no longer capable of effectively utilizing the HC and CO components in the exhaust gas for the reduction of NOx, and it becomes difficult to efficiently reduce the NOx.
Further, when the moment of terminating the rich spike operation is judged based on the output of the air-fuel ratio sensor downstream of the catalyst as in the device of the above JP-A-2003-56379, the HC and CO components arrive at the air-fuel ratio sensor in relatively large amounts due to blow-by even though the reduction of NOx has not actually been completed on the NOx occluding and reducing catalyst, and the air-fuel ratio that is detected often reaches a judging value.
In this case, the rich spike operation terminates before the NOx occluded in the NOx occluding and reducing catalyst is all reduced and purified. Therefore, the operation at a lean air-fuel ratio is resumed again in a state where the NOx occluding and reducing catalyst is still occluding the NOx in relatively large amounts and this causes a problem of a decrease in the NOx occluding capability of the NOx occluding and reducing catalyst.
In the supercharged lean burn engine, further, the amounts of air in the cylinders are large during the operation at a lean air-fuel ratio, and the fuel must be supplied in increased amounts for conducting the operation at a rich air-fuel ratio.
On the other hand, the rich spike operation usually starts when the amount of NOx occluded in the NOx occluding and reducing catalyst reaches a predetermined value. Therefore, the amounts of HC and CO required for reducing all of the occluded NOx become nearly constant.
The rich spike operation terminates at a moment when the HC and CO components are supplied in the above-mentioned amounts to the NOx occluding and reducing catalyst, while the amounts of HC and CO components used for reducing the NOx in the exhaust gas are determined depending upon the air-fuel ratio of the exhaust gas. That is, if an air-fuel ratio of the exhaust gas is defined as the ratio of the amount of the air taken in by the engine to the amount of the fuel supplied to the engine, then, a difference between the stoichiometric air-fuel ratio and the air-fuel ratio of the exhaust gas during the rich spiking (hereinafter called “richness degree”) varies in proportion to the concentration of excess of HC and CO components in the exhaust gas (amounts of the HC and CO components contained in the exhaust gas without burning).
Therefore, the amounts of HC and CO that can be used for reducing the NOx during the rich spiking, as a whole, become a value obtained by multiplying the richness degree of air-fuel ratio during the rich spike operation (difference between the stoichiometric air-fuel ratio and the air-fuel ratio during the rich spike operation) by the flow rate of the exhaust gas.
During the supercharged lean burn operation as described above, the flow rate of exhaust gas of the engine becomes considerably large. The total amount of the HC and CO components necessary for reducing the NOx occluded in the NOx occluding and reducing catalyst, on the other hand, remains nearly constant.
In the supercharged lean burn engine, therefore, the time for sustaining the rich spike operation that is required becomes much shorter than that in the case of the naturally-aspirated engine.
In the supercharged lean burn engine in practice, however, the flow rate of the exhaust gas is large, the SV value through the NOx occluding and reducing catalyst is high and, therefore, the HC and CO components in the exhaust gas cannot be efficiently used. Accordingly, the time for sustaining the rich spike operation that is really required becomes considerably longer than the time that is usually required and, this causes a decrease in the fuel efficiency of the engine.
When the NOx occluding and reducing catalyst is used for the device for purifying exhaust gases of the supercharged lean burn engines, execution of the rich spike operation in a traditional manner causes a problem of decrease in the NOx reduction efficiency.
In the foregoing was described the case of the supercharged lean burn engine. However, in the case of the naturally aspirated lean burn engine, too, though the intensity of the problem may be different, the problem of blow-by occurs when the amount of the air taken in increases and a problem of a decrease in the NOx reduction efficiency during the rich spike operation occurs.